1. Field
The present invention generally relates to a vehicle drive force distributing apparatus suitable for a transfer case of a four-wheel drive vehicle.
2. Related Art
A drive force distributing apparatus including a first roller mechanically coupled to a transmission system of a pair of main drive wheels and a second roller mechanically coupled to a drive system of a pair of sub-drive or subordinate wheels is disclosed. The system becomes engaged when first and second rollers are mechanically pressed so as to make contact with each other at their outer peripheral surfaces. As such, a mechanical torque can be distributed between the main drive wheels and subordinate drive wheels such that a percentage allocated to the main drive wheels vs. the percentage allocated to the subordinate drive wheels is continuously selectable. Accordingly, a torque transmission capacity between the rollers can be controlled by adjusting a radial pressing force between the first roller and the second roller so as to adjust the distribution of the drive force between the main drive wheels and the sub-drive wheels.
An example mechanism for performing this drive force distribution control is disclosed in the Japanese Laid-open Patent Publication No. 2011-11560 (and corresponding U.S. Patent Application Publication No. 2012/0100955 A1). In this example a second roller is supported in an eccentric shaft portion of a crankshaft and the rotation axis of the second roller revolves or turns about the eccentric axis by operating the crankshaft to rotate. By displacing the position of the rotation axis of the second roller (by an angular amount along a curve) the second roller becomes displaced toward the first roller. Thus, the radial pressing force between the first roller and the second roller may be controlled. To perform this control, it is necessary to detect the rotation angle of the crankshaft. The rotation angle of the crankshaft corresponds to the angular position of the eccentric axis of the second roller and is obtainable as an amount of angular movement with respect to a reference position.
The reference point is set as follows. The crankshaft is turned and the axis of the second roller is therefore displaced toward the first roller. Meanwhile the first roller is turned to one direction at a constant torque while the second roller is turned in the reverse direction at the same magnitude of constant torque. As the first and second rollers are pressed with increasing force toward one another (by turning the crank shaft) eventually the second roller stops turning due to the torque imposed by the first roller. The angular position of the crankshaft at which the second roller stops it taken as a reference point. This reference point corresponds to the situation in which the two rollers are pressed together with sufficient force so prevent slippage between the rollers and thus represents the point at which drive force distribution system is beginning to be engaged for transferring torque. By increasing the crankshaft rotation angle beyond this reference point allows torque to be increasingly diverted from the main drive wheels to the sub-drive wheels.
The above described system, however, suffers from the drawback that, generally, a time delay is associated with the detection of the rotation angle of the crankshaft.